Raw natural gas must be treated prior to its liquefaction for several reasons. These include removing compounds which interfere with the liquefaction process, with the separation and recovery of hydrocarbon liquids and with meeting the specifications set for the recovered products. For example, the gas must be dried to prevent ice formation during cryogenic operations. Hydrogen sulfide ordinarily must be removed because of its toxic nature. A large number of commercial processes are in use for treating and separating of raw wellhead gas. The steps used in these different processes are each well known to those skilled in the art.
Some natural gas contains mercury at levels as high as 200 to 300 micrograms per cubic meter. For example, the mercury level of natural gas produced at one field in Indonesia is about 250 micrograms per cubic meter. Concentrations of mercury at this level creates safety hazards and air pollution problems. Refinery equipment such as heat exchangers can be adversely effected by the action of accumulated mercury. The problem of mercury in natural gas is discussed further in U.S. Pat. No. 4,094,777 and French Pat. No. 2,310,795, both of which are incorporated herein by reference.
Crude natural gas containing mercury ordinarily is treated by first flowing it through a bed containing sulfur distributed over a carbon support. The free sulfur present reacts with mercury in the natural gas and removes it from the natural gas. The gas is then contacted with an alkali carbonate to remove the carbon dioxide and hydrogen sulfide present in the gas and subsequently is treated by liquid amine extraction to remove any residual hydrogen sulfide. The gas is then dehydrated to remove water and finally is cooled and liquefied after treatment in a heat exchanger. It is the heat exchange equipment which is a primary source of problems resulting from mercury contamination. Ordinarily the heat exchangers are made of aluminum which is easily corroded and ultimately destroyed by the cumulative effect of mercury present in the natural gas.
Although the mercury can be removed by contact with the sulfur-on-carbon absorbent, the mercury content can be lowered only to a level of from 250 to 0.03 micrograms per cubic meter. This lower level is considered to be the minimum concentration achievable under the prevailing thermodynamic limitations. As the mercury removing system ages, however, the mercury level in the effluent gas will increase up to 0.1 micrograms per cubic meter or higher over a number of years. The mercury content thus may reach levels which are considered too high for the continued safe operation of the aluminum heat exchangers. This is because the mercury tends to condense on the cold surfaces of the heat exchanger and there to react with the aluminum leading to its ultimate corrosion and failure.
Furthermore operating experience has shown that the mercury-removing equipment upstream (e.g. sulfur-on-carbon absorbent beds) sporadically malfunctions. Consequently mercury in the natural gas is not removed but is carried through the gas system to a point where it contacts aluminum equipment such as the heat exchangers. This malfunction of the upstream mercury removal equipment has been found to contribute significantly to the overall mercury corrosion problem.
Even though the mercury does not immediately react with the aluminum it may still tend to accumulate on the surface of the aluminum and where the aluminum is not protected by a shield of aluminum oxide or if the aluminum oxide coating becomes scratched, the mercury will further react with the aluminum.
A primary purpose of this invention therefore is to provide a method of passivating, or rendering non-reactive, residual mercury present in gas liquefaction equipment. Still another object of this invention is to prevent the further corrosion and deterioration of gas liquefaction equipment that has become contaminated by the accumulation of mercury.